1. Technical Field
The embodiments discussed herein relate to a magnetic head with the function of inducing magnetization reversal by applying a high-frequency magnetic field to a perpendicular magnetic recording medium, a magnetic recording apparatus including the magnetic head, a magnetic head drive control apparatus, and a method for controlling the same.
2. Related Art
As a result of the evolution of the Internet environment and the increase in the number of data centers spurred by the increasingly widespread use of cloud computing, the amount of information that is generated is rapidly increasing. There seems no doubt that magnetic recording apparatuses, such as magnetic disk drives (HDD) that have the highest recording density and superior bit cost, will continue to be the major storage device in the “age of big data”. Thus, the capacity of the magnetic recording apparatus needs to be further increased and higher recording density needs to be achieved for that purpose.
Transition from the longitudinal magnetic recording based on a ring-type longitudinal recording head and a longitudinal magnetic recording medium to the perpendicular recording began in 2005. Increases in recording density have been achieved mainly by the magnetic recording head of a main magnetic pole and magnetic shield pole type and by improvements in perpendicular recording performance enabled by a perpendicular magnetic recording medium referred to as an ECC (Exchange Coupled Composite) medium whose ease of recording was significantly increased by decreasing the crystalline magnetic anisotropy energy Hk on the surface layer (also called “cap layer”) side of the magnetic layer. Achieving higher recording density is basically governed by scaling laws, requiring decreases in the track width of the main magnetic pole of a perpendicular magnetic recording head, a head-to-medium magnetic spacing, and the crystallographic grain of the ECC-type perpendicular magnetic recording medium, for example. However, a decrease in the crystallographic grain of the perpendicular magnetic recording medium leads to a decrease in anisotropic energy that would maintain a magnetization state, so that the recorded magnetization state tends to be more readily disturbed by thermal fluctuation. This phenomenon is referred to as a super-paramagnetic effect. Thus, there is supposedly a practical limit to simply extending current technology in the era on the order of 1 Tb/in2, as discussed by Non-patent Document 1. This practical limit may be referred to as “super-paramagnetic limit” or “trilemma”.
Developing a technology for overcoming the limit is a major problem in achieving an increase in recording density. Patent Document 1 proposes a spin heating recording method whereby even a high-coercive force medium can be written with a low magnetic field without virtually any increase in medium temperature by utilizing an effective decrease in coercive force due to the absorption of the energy of a high-frequency magnetic field satisfying magnetic resonance conditions by the spin of the magnetic recording medium, the high-frequency magnetic field being supplied to the magnetic recording medium by causing an external high-frequency source to track the movement of the magnetic head. In such a spin heating recording method, because the high-frequency electromagnetic field is applied such that the power is concentrated in a frequency range satisfying the magnetic resonance conditions, the internal degrees of freedom other than spin, such as the plasma vibration and lattice vibration of conduction electrons, are not much excited and only spin can be selectively excited, so that the temperature of the medium as a whole is not increased as would happen by conventional heating. Such a method for increasing recording density by exciting the precession of medium magnetization by applying a microwave-band high-frequency magnetic field to the magnetic recording medium, and magnetically recording information in the perpendicular magnetic recording medium with large magnetic anisotropy energy while decreasing a switching field, is referred to as MAMR (Microwave Assisted Magnetic Recording) (Non-patent Document 1).
In recent years, a practical spin torque oscillator (STO) of a fine structure utilizing a high-frequency magnetic field generation layer (FGL) such that a high-frequency magnetic field is generated by causing a high spin rotation (precession) by spin torque has been proposed, as discussed in Patent Documents 2 and 3 and Non-patent Document 2. Further, Patent Document 4 discloses a method for inducing magnetization reversal with improved efficiency by causing a high-frequency magnetic field oscillator to generate a high-frequency magnetic field (circular polarization magnetic field) rotating in the same direction as the direction of precession of the magnetization of the magnetic recording medium for magnetization reversal in accordance with the recording field polarity. Also, research and development for practical application of the microwave assisted magnetic recording with the above-described features are recently actively being pursued. For example, Patent Documents 3 and 5 propose magnetic heads such that stable and high-frequency magnetic field assisted recording is enabled by adopting a structure in which the STO with low coercive force is disposed adjacent to the main magnetic pole of a perpendicular magnetic head, or between the main magnetic pole and an additional magnetic shield (auxiliary or return pole). These publications also disclose a structure in which the STO is disposed on a medium travel direction side of the main magnetic pole or on the opposite side, and a structure in which the direction in which STO films are layered is substantially perpendicular or parallel to the direction of movement of the medium.
Thus, with regard to the microwave assisted recording, studies have been made on the premise that an increase in recording density is made as an extension of the perpendicular recording technology that has been put to practical application since 2005 with areal density of 133 Gb/in2. The ring-type longitudinal recording head that had been used for longitudinal magnetic recording, in which a ring-type (C-shaped) magnetic core is formed and recording is performed with a strong longitudinal recording field produced from a write gap of the magnetic core, was considered to be not suitable for increasing recording density for perpendicular magnetic recording due to a small write field gradient. Thus, the microwave assisted recording heads proposed by the related art have been based on the perpendicular magnetic recording head (main magnetic pole and magnetic shield pole type recording head) that is currently widely used as a standard perpendicular recording head and that performs recording by using a strong perpendicular recording field emitted from a main magnetic pole.
Patent Document 1: JP 7-244801 A
Patent Document 2: JP 4677589 B
Patent Document 3: U.S. Pat. No. 7,616,412 B2
Patent Document 4: JP 4255869 B
Patent Document 5: JP 2009-70541 A
Patent Document 6: JP 2007-220232 A
Non-patent Document 1: Y. Shiroishi, et al., “Future Options for HDD Storage”, IEEE Trans. Magn., Vol. 45, No. 10, pp. 3816-3822 (2009)
Non-patent Document 2: X. Zhu and J.-G. Zhu, “Bias-field-free microwave oscillator driven by perpendicularly polarized spin current”, IEEE Trans. Magn., Vol. 42, pp. 2670-2672 (2006)
Non-patent Document 3: Y. Tang and J.-G. Zhu, “Narrow Track Confinement by AC Field Generation Layer in Microwave Assisted Magnetic Recording”, IEEE Trans. Magn., Vol. 44, pp. 3376-2379 (2008)
Non-patent Document 4: J-G. Zhu, X. Zhu and Y. Tang, “Microwave Assisted Magnetic Recording”, IEEE Trans. Magn., Vo. 144, No. 1, pp. 125-131 (2008)